Method for producing patent leather



Dec. 24, 1968 w. H. HoGG ETAL A3,418,188

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United States Patent O 3,418,188 METHOD FOR PRODUCING PATENT LEATHER Wilfrid Henry Hogg, Sutton Coldfeld, and Peter Ford, Erdington, Birmingham, England, assignors to Dunlop Rubber Company Limited, London, England, a British company Filed May 1, 1963, Ser. No. 277,105 Claims priority, application Great Britain, May 9, 1962, 17,762/ 62 14 Claims. (Cl. 156-246) This invention relates to coa-ted materials and particularly to a method of manufacture of patent leather.

Patent leather is understood to be leather which is provided with a high gloss surface, and "hitherto, such leather has been manufactured by treating leather with a varnish based on linseed oil or nitrocellulose. One of the main diiiiculties of this process has been the necessity to use a solvent to reduce the viscosity of the varnish in order that it may be easily applied to the surface of the leather and the solvent has tended to extract nautral fats land oils from the leather which penetrate into the varnish lm with the result that the surface of the finished patent leather h-as been inferior.

According to the present invention patent leather comprises leather having adhered to one surface thereof a coating of a cured polymer composition.

According to the present invention also, a method for the manufacture of patent leather comprises -applying a layer of a curable, liquid polymer composition to a smooth supporting surface, partially curing said composition, applying leather to the partially-cured film so obtained, completing the cure of said lm, yand releasing the cured film adhering to the leather from the supporting surface.

The method of the present invention provides `a convenient and easy way of obtaining patent leather in which the leather is provided with a surface coating of a film of a polymer composition which is strongly adhered to the surface of the leather. The curable, liquid polymer composition should be partially-cured before the leather is applied to such an extent that it is not deformed to any great extent when the leather is applied thereto, and should also have a sufficient degree of tackiness to adhere to the surface of the leather.

The method of the invention can be used for coating leather with 'any liquid polymer which can be cross-linked to a solid polymer or an elastomer. The polymer composition which is used is rst applied to a smooth supporting surface, `and to enable this to be easily applied, the composition should be a liquid so that it can be extruded through an orifice, sprayed `on to the surface, or bruhsed on to the surface. Typical liquid polymers which can be Iused to form compositions, to be used in the method of the invention, are liquid polybutadieue, liquid copolymers of styrene and butadiene, liquid chloroprene and the liquid polysulphide rubbers. The liquid polymer is mixed with the appropriate curing agents prior to application to the supporting surface 'and with any other ingredients as may be desired such as pigments, antioxidants, and accelerators. Preferably, the liquid polymer composition is a polyurethane precursor such as a liquid prepolymer obtained by reacting a polyether, polyester, polyesteramide, or polyamide with an organic polyisocyanate or polyisothiocyanate, to give a reaction product containing at least two terminal or substantially-terminal -NCX groups, wherein X represents an atom of oxygen or an atom of sulphur. A process for producing 'a typical composition based on a polyurethane prepolymer which can be used, Watson and Hartle, Ser. No. 137,750, tiled Sept. 13, 1961, and now abandoned. This application describes and claims a process for producing a polyurethane which comprises mixing under anhydrous or substantially-anhydrous conditions, a reaction product containing at least two terminal or substantially-terminal groups in the molecule, each group comprising an aliphatic radical containing an isocyanate substituent, with -a -polyol containing more than two -OH or SH groups in the molecule and a catalyst to accelerate cross-linking in said reaction product. The reaction product described in this co-pending patent application can be produced by reacting a polymeric diol with an organic aliphatic polyisocyanate, but reaction products produced from aromatic polyisocyanates can also be used in the present invention. Generally, it will be necessary to mix the liquid polymer composition, when based on a polyurethane prepolymer, with a cross-linking agent and catalyst immediately prior to application to the supporting surface. Typical cross-linking agents which can be used, depending `on the nature of the liquid polyurethane composition, are organic compounds con-taining active hydrogen atoms such as polyols, e.g. triols.

Alternatively, the liquid polymer composition can comprise a mixture of an organic compound containing at least two groups each containing an active-hydrogen atom and a monomeric organic polyisocyanate or polyisothiocyanate, which mixture is allowed to react to form a polyurethane after application to the supporting surface. In this case, the reactants must be mixed together immediately prior to application of the mixture to the supporting surface.

If desired, the polymer composition can be dissolved in an organic solvent therefor to enable it to be applied easily to the supporting surface, and when such a solution is applied, then it is ldesirable to ensure that substantially all the solvent is evaporated from the polymer composition before the leather is applied to the film. Preferably, therefore, the solvent should not have a low volatility and can be, for example, methylene chloride, acetone, or a volatile ester. Generally, the thickness of the iilm to which the leather is applied will be from 0.002 inch to 0.006 inch.

The liquid polymer composition supported on the smooth supporting surface is then partially-cured and this may be effected, `depending on the nature of the polymer composition, at room temperature or at yan elevated temperature, or by irradiation. The composition can be subjected to irradiation from a radiation source such as cobalt 60 and this is particularly desirable when liquid polyethylene or liquid polyethylene/ propylene copolymers form the basis of the liquid polymer composition. The liquid polymer composition, depending on its nature and the curatives `or cross-linking agents employed, may become partially-cured at room temperature, and polyurethane prepolymers can be allowed to cross-link in this way, although in some cases, it may be preferable to heat them to yaccelerate the cross-linking reaction.

The supporting surface to which the liquid composition is applied should be such that it does not bond or adhere to `any great extent to the polymer composition when finally cured, `and it may comprise, for example, a substantially impervious material such las polytetrauorethylene, a polymer of vinyl chloride, polyethylene or polypropylene. The supporting surface should also be smooth and preferably highly polished, in order that the completed patent leather has a high gloss-surface, and for this reason, it is preferred to use as the supporting surface, a sheet of polished plate-glass.

When the liquid polymer composition is partially-cured, the leather to be coated with the ilm of the polymer composition is applied to the surface of the partiallycured polymer composition. This is normally achieved by applying pressure to the leather by means, for example, of a roller, to ensure that any air, Which may be trapped between the film and the leather, escapes, otherwise this will damage the resultant product. The leather can `be anyV type of leather such as natural leather, i.e. that obtained from hides of cattle, or can be artificial leather such as that described in co-pending application of Goy et al., Ser. No. 176,051, filed Feb. 27, 1962 and now abandoned. After the leather has been applied to the film of partially-cured composition, the assembly is fully cured by further heating or irradiation or by any other suitable means. When the curing has been completed, the leather, with the film of cured polymer composition adhered thereto, is removed from the supporting surface to give a leather coated on one surface with the cross-linked polymer composition. This can conveniently be effected by soaking the leather in water, after which treatment the coated leather can be easily stripped from the supporting surface. The surface of the cured polymer composition has a high gloss and the surface coating is strongly adhered to the leather. The temperature to which the assembly is heated to complete full cure should not be sufficient to cause damage to the leather and it is preferred, when a polyurethane composition is used to form the film, to heat to a temperature of from 40 C. to 80 C. to complete the curing of the composition.

The method of the present invention allows a wide variety of patent leathers to be manufactured, for example, the polymer composition can be a clear composition and can rbe applied to leather of a variety of colours, or the polymer composition itself can contain finelydivided pigments, or dyes. In this way, a variety of coloured patent leathers can be obtained. The abovementioned vco-pending application Ser. No. 137,750 describes a polyurethane composition which does not discolour when exposed to strong sunlight and if such a composition containing a white pigment is employed in the method of the present invention, then a white patent leather is obtained which does not discolour when exposed to strong sunlight. Alternatively, the composition can -be of a pastel shade which will also be non-discoloured by exposure to light.

The process of the invention is schematically illustrated in the accompanying drawings in which no attempt has been made to illustrate the various features to scale.

In FIG. l a thin layer of liquid polymer composition has been extruded from orifice 6 into a shallow dish 7 in the bottom of which is a smooth surface 8.

In FIG. 2 the polymer composition 5 has been allowed to partially cure and any solvent present has been evaporated so that the polymer layer is firm but tacky and the leather 9 can now be applied.

In FIG. 3 the leather 9 has been applied and the cure of the polymer composition 5 is completed in order to bond the leather and polymer together.

FIG. 4 represents the finished product which has a layer of polymer bonded to the leather and has a high gloss finish on the outer surface 10 of the polymer layer.

The method for the production of patent leather, according to the present invention, can be carried out rapidly and with considerably less labour than processes used hitherto, to yield products having a uniform high gloss and uniform colour. The method does not require to be operated in an absolutely dust-free atmosphere, since the surface of the patent leather carrying the high gloss is protected by the smooth supporting surface. The polymer compositions are formulated to give coatings of reproducible, predetermined flexibility and the patent leathers retain their exibility during long periods of storage and service. The method enables lightcoloured or white patent leathers to be obtained which do not discolour during exposure to strong sunlight.

The invention is illustrated in the following examples:

' EXAMPLE I A polyurethane prepolymer was prepared by the following technique. 1,000 grammes of a neopentyl adipate ester having a hydroxyl number of 80 and an acid number of 2, were heated for 2 hours at 120 C. under a vacuum to remove residual moisture, and then cooled to C. The ester was then mixed with 579 grammes of dicyclohexylmethane diisocyanate and the temperature rose to C. and was maintained at this value for 2 hours. The prepolymer syrup so obtained was stored in sealed tins until required. After storing for l day, the prepolymer was found to contain 7 percent by weight of free isocyanate groups.

The following solutions were made up.

(A) 10 grammes of the prepolymer syrup were dissolved in methylene chloride and made up to 100 mls. by adding a further quantity of methylene chloride.

(B) 0.575 gramme of trimethylol propane was dissolved in 10 mls. of acetone and made up to 100 mls. by adding a further quantity of acetone.

(C) 0.37 gramme of dibutyl tin dilaurate was dissolved in methylene chloride and made up to 100 mls. by adding a further quantity of methylene chloride.

These solution were stable when stored in stoppered asks. Immediately prior to use, a mixed solution was made up from 15 mls. of solution A, 25 mls. of solution B, and 1 ml. of solution C. The mixed solution so obtained was cast on to a fiat glass plate fitted with raised sides to form a shallow dish whose internal dimensions were 6 inches by 6 inches. The plate was held rigid on a supporting base so that its upper surface was level. After the mixed solution had been cast on to the glass plate, it was allowed to stand at room temperature to allow the solvent to evaporate, and after 35 minutes, no trace of solvent could be detected by smell, and the film was extremely tacky.

A piece of undressed leather 5 inches by 5 inches, suitable for use in the manufacture of shoe-uppers and which had been dyed black, was carefully pressed on to the film. Small pockets of trapped air were allowed to escape through the leather, and then, after leaving for one hour at room temperature, the plate with the leather adhering to the film was placed in a hot air oven at 70 C. for 1 hour. At the end of this time, the assembly was allowed to cool and the leather was soaked in water and was then easily stripped from the plate. The thickness of the film adhering to the leather, was approximately 0.0025 inch and the surface of the composition had a high gloss.

A strip of the coated leather was then flexed using a De Mattia flexing machine, in which the clamps gripping each end of the test piece approached each other to a distance of 3 cm. and separated to a distance of 8.7 cm. After one million fiexing cycles, no cracking of the surface coating was observed.

EXAMPLE II This example illustrates the coating of undyed leather with a black glossy lm of a polyurethane composition.

Two grammes of a prepolymer syrup prepared as in Example I were dissolved in 50 millilitres of methylene chloride, in which 0.06 gramme of a mixture of spiritsoluble black dyes which are non-reactive to isocyanate groups, had already been dissolved. The black dyes used are commercially available under the names Ceres black BN and Ceres black G. These dyes were used in the proportion of 2 parts of Ceres black BN to 1 part of Ceres black G. Three millilitres of a solution of trimethylol propane in acetone, containing 5 grammes of trimethylol propane per 100 millilitres acetone, and 1 millilitre of a solution of dibutyl tin dilaurate in methylene chloride, containing l gramme of dibutyl tin dilaurate per 100 millilitres of methylene chloride were added and the solution so obtained was thoroughly mixed.

The mixed solution was cast on to a flat glass plate as in Example I and the solvent allowed toevaporate at room temperature.

A piece of undressed leather, 5 inches x 5 inches, suitable for use in the manufacture of shoe-uppers, but which had not been dyed, was pressed onto the film as in Example I. After a further hour at room temperature, the plate with the leather adhering to the film was placed in a hot air oven at 70 C. for one hour then allowed to cool. The assembly was soaked in water and the leather with the film adhering to it was easily stripped from the plate. The leather was allowed to dry out at room temperature.

The leather had been given a completely black, highly glossy surface, and although the hn was only about 0.004 inch thick, no trace of the natural colour of the leather could be seen through it.

EXAMPLE III Undyed leather was provided with a White glossy surface in the following manner.

120 grammes of dry titanium dioxide (rutile) were incorporated, using a paint mill, into 300 grammes of a neopentyl adipate polyester having a hydroxyl number of 100. 300 grammes of the resultant white paste were heated for 2 hours at 120 C. under vacuum to remove residual moisture, and then cooled to 70 C. 150 grammes of dicyclohexyl methane diisocyanate Were mixed into the pigmented polyester. The temperature rose to 100 C. and the reaction mixture was maintained at this temperature for 2 hours. The prepolymer was stored in a sealed tin, and shortly before use was found by analysis to contain 6.85 percent by weight of free isocyanate groups.

6 grammes of the pigmented prepolymer were dispersed in 50 millilitres of methylene chloride and mixed with (a) 8 millilitres of a solution of 5 grammes of trimethylol propane in 100 grammes of acetone, and (b) 1.5 grammes of a solution of 1 gramme of dibutyl tin dilaurate in 100 grammes of methylene chloride. The mixed solution was cast onto a glass plate, 6 inches x 6 inches, as in the previous examples, and when the solvent had evaporated at room temperature, a piece of undressed, undyed leather, 5 inches x 5 inches, was applied to the film as in Examples I and II. The plate, with the leather adhering to the film, was allowed to remain at room temperature overnight, after which time the film was fully cured.

The leather with the film adhering to it was removed from the plate as in Examples I and II and the leather dried at room temperature. The leather had a white opaque highly glossy surface.

Part of this sample was kept in the dark, and another portion was exposed to intense ultra-violet light. At the end of 6 weeks the two portions were examined. No difference was apparent between the samples in colour or other respect.

A further portion was exposed to natural daylight for a period of two months, again without suffering any change in appearance.

EXAMPLE IV A polyurethane prepolymer was prepared by the following technique. 300 grammes of a polyneopentyl adipate ester having a hydroxyl number of 100 and an acid number of 2.6 were melted at 70 C., and 213.5 grammes of dicyclohexyl methane diisocyanate were stirred into the molten ester. The mixture was reacted at 100 C. for 2 hours, cooled and then stored at room temperature in sealed tins. Shortly before using, this prepolymer was found to contain 8.1 percent by weight of free isocyanate groups.

2 grammes of this prepolymer were dissolved in 8.75 millilitres of acetone and thoroughly mixed with (a) 0.87 millilitre of an acetone solution of trirnethylol propane containing grammes of trimethylol propane per 100 millilitres of solution and (b) 0.2 millilitres of an acetone suspension of dibutyl tin dilaurate per 100 millilitres of suspension. The mixed solution so obtained was cast on to a llat glass plate as in Example I, and the solvent was allowed to evaporate. A piece of undressed, undyed leather, 5 inches x 5 inches, was applied to the film as in Example I, and the plate with the leather adhering to the film was pressed in a hydraulic press at 70 C. for 35 minutes at a pressure of 50 pounds per square inch, the glass plate being protected from cracking by foam rubber. The assembly was allowed to cool under pressure, and the leather with the film adhering thereto was stripped from the glass plate, after soaking in water as in Example I. The thickness of the film was approximately 0.004 inch, and the surface of the composition had a high gloss.

EXAMPLE V This example illustrates the use of an aromatic diisocyanate.

A polyurethane prepolymer was made from an aromatic diisocyanate in the following manner. 31.6 grammes of toluene diisocyanate were stirred at room temperature, and grammes of a molten polyneopentyl adipate ester (as in Example IV), maintained at 40 C., were added to the isocyanate over a period of half an hour. The reaction product was then stored in a sealed tin at room temperature, and the next day was found to contain 5.7 percent by Weight of free isocyanate groups.

To prepare the film, the following solutions were mixed:

(a) 5 millilitres of an acetone solution of the prepolymer containing 40 grammes of prepolymer per 100 millilitres of solution.

(b) 0.61 millilitre of an acetone solution of trimethylol propane containin 7 20 grammes trimethylol propane per 100 millilitres of solution.

(c) 0.5 millilitre of an acetone solution of Ceres black BN containing 8 grammes of Ceres black BN (see EX- ample II) per 100 millilitres of solution.

(d) 0.25 millilitre of an acetone solution of Ceres black G containing 8 grammes of Ceres black G per 100 millilitres of solution.

(e) 0.2 millilitre of acetone suspension containing 5 grammes of dibutyl tin dilaurate per 100 millilitres of suspension.

3 millilitres of acetone were added. The following operations were carried out as in Example 1V; the solution was cast, and the solvent allowed to evaporate; the leather was applied, and the laminate was pressed; the leather with the film adhering thereto was stripped from the plate, after soaking in water.

The laminate had an attractive black colour and high gloss.

EXAMPLE VI This example illustrates the preparation of a patent leather having high scuff resistance.

400 grammes of a polyepichlorohydrin (hydroxyl number 61) was heated for 2 hours at 120 C. under vacuum to remove residual moisture and then cooled to 100 C. 75.7 grammes of toluene diisocyanate were added with stirring in an atmosphere of nitrogen, and the mixture reacted for 20 hours at 100 C. The prepolymer so obtained contained 4.35 percent by weight of reactive isocyanate groups.

350 grammes of this prepolymer was dissolved in 200 grammes of ethyl acetate, and added slowly, under an atmosphere of nitrogen, and with continuous stirring, to a solution of 20.4 grammes of butane diol in 250 grammes of ethyl acetate. The solvent was then distilled off at 100 C., leaving a semi-solid reaction product with no reactive isocyanate groups. The following solutions were made up:

(a) An acetone solution of this reaction product containing 23.7 grammes of reaction product per 100 milliliters of solution.

(b) A solution containing 24 grammes of triphenyl methane triisocyanate in a mixture of 96 grammes of methylene chloride and grammes of trichloroethylene.

(c) An acetone solution containing 8 grammes of Ceres black BN per 100 milliliters of solution,

(d) An acetone solution containing 8 grammes of Ceres black G per 100 mls. of solution.

Immediately prior to use, a mixed solution was made up, consisting of 14 millilitres of solution (a), 5.5 millilitres of solution (b), 0.8 millilitre of solution (c), and `0.4 millilitre of solution (d). The mixed solution was cast on a glass plate, and the solvent allowed to evaporate in the manner of the foregoing examples. A piece of undressed, undyed leather was then applied and the laminate pressed and cured in the manner of Examples IV and V. When stripped from the plate, the leather had a very attractive black colour and high gloss.

A piece of this patent leather was tested for scuffresistance by the dry rope abrasion test and withstood 20 cycles without showing surface failure. On the same test, the glossy surface of a piece of commercial patent leather made by coating natural leather with PVC, was abraded down to the leather in one cycle.

A strip of this patent leather was then flexed using a De Mattia flexing machine, as in Example I. After one million flexing cycles, no cracking of the surface coating was observed.

Having now described our invention, what We claim is:

1. A method for the production of patent leather which comprises applying a layer of a curable, liquid, polymer composition to a smooth supporting surface, partially curing said composition, applying leather to said partially cured lm so obtained, completing the cure of said lm, and releasing the cured lilm adhering to the leather from said supporting surface.

2. A method according t claim 1 in which the liquid polymer composition is extruded on to the supporting surface.

3. A method according to claim 1 in which the liquid polymer composition is dissolved in `an organic solvent therefor, and the solvent is evaporated from the composition before the leather is applied thereto.

4. A method according to claim 3 in which the solvent is a member of the class consisting of methylene chloride, acetone and volatile esters.

5. A method according to claim 1 in which the supporting surface comprises a member of the class consisting of polytetrafluoroethylene, polymers of vinyl chloride, polyethylene and polypropylene.

6. A method according to claim 1 in which the liquid Ipolymer composition is partially cured at room temperature.

7. A method according to claim 1 in which the polymer composition is partially cured at a temperature of 40 C. to 80 C.

8. A method according to claim 6 in which the final cure of the liquid polymer composition is etected at a temperature of 40 C. to 80 C.

9. A method according to claim 1 in which the cured film of polymer composition adhering to the leather is soaked in Water to enable the coated leather to be easily stripped from the supporting surface.

10. A method according to claim 1 in which the liquid polymer composition is a liquid polyurethane-forming composition.

11. A method according to claim 10 in which the liquid polyurethane-forming composition comprises a mixture of an organic compound containing at least two groups each containing an active hydrogen atom and an organic compound containing at least two terminal or substantially terminal groups each containing an -NCX radical wherein X represents an atom of the group consisting of oxygen and sulphur.

12. A method according to claim 10 in which the liquid polyurethane-forming composition comprises a mixture 0f a. liquid prepolymer, comprising a reaction product of an organic compound containing at least two l,groups each containing an active hydrogen atom and an organic compound containing at least two terminal or substantially-terminal groups each containing an -NCX radical wherein X represents an atom of the class consisting of oxygen and sulphur, and a cross-linking agent for said polyurethane-forming composition.

13. A method according to claim 11 in which a catalyst is added to the polyurethane-forming composition.V

14. A method according to claim 1 in which the liquid polymer composition is a member of the class consisting of liquid polybutadiene, liquid copolymers of styrene and butadiene, liquid chloroprene, and liquid polysulphide rubbers.

References Cited UNITED STATES PATENTS 1,824,690 9/1931 Schneider 156-231 2,119,727 6/1938 Talbot 264-213 3,009,847 11/1961 Alles et al 156-231 3,054,716 9/1962 Bergstein et al. 264-212 3,030,249 4/ 1962 Schellenberger et al. 161--190 3,298,856 1/1967 Harding 117-142 X FOREIGN PATENTS 540,530 ll/ 1941 Great Britain.

EARL M. BERGERT, Primary Examiner.

T. R. SAVOIE, Assistant Examiner.

U.S. CI. X.R. 

1. A METHOD FOR THE PRODUCTION OF PATENT LEATHER WHICH COMPRISES APPLYING A LAYER OF A CURABLE, LIQUID, POLYMER COMPOSITION TO A SMOOTH SUPPORTING SURFACE, PARTIALLY CURING SAID COMPOSITION, APPLYING LEATHER TO SAID PARTIALLY CURED FILM SO OBTAINED, COMPLETING THE CURE OF SAID FILM, AND REALEASING THE CURED FILM ADHERING TO THE LEATHER FROM SAID SUPPORTING SURFACE. 